Spark-plug.



F. M. FURBBR.

SPARK PLUG.

APPLIGATION FILED JULY 5, 1913.

1,126,974. Patented Feb. 2, 1915.

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UNITED STATES PATENT OFFTGEQ FREDERICK M. FURBER, 0F REVERE, MASSACHUSETTS, ASSIGNOR ".10 ARTHUR E.

MOSLER, OF NEW YORK, N.

SPARK-PLUG.

Specification of Letters Patent.

Application filed July 5, 1913. Serial No. 777,578.

To all whom it may concern Be it known that I, FREDERICK M. FURBER, a citizen of the United States, residing at Revere, in the county of Sufiolk and Commonwealth of Massachusetts, have invented certain Improvements in Spark-Plugs, of which the following description, in connection with the accompanying drawings, is a specification, like reference characters on the drawings indicating like parts in the several figures.

This invention relates to s'park'plugs of the character commonly used in internal combustion engines.

Spark plugs usually comprise an outer body or shell adapted to be secured in the cyhnder of an engine, electrodes electrically lator, and some means sulator in the shell.

an insulator, a pair of separted by theinsufor clamping the in- The insulators used in necessarily made of material that renders them comparatively fragile and the frequent breakage of these parts is one of the chief elements of trouble experienced in operating engines of this type. This result is caused partly by the fact that r the plugs are subjected to very rough usage,

' the construction of the but is due in a large measure to the mannerin which theinsulators are secured in their shells.

The present invention is concerned particularly with the mounting of the insulator in the shell and it aims to simplify plug and to provide a construction that will materially lessen the liability of breaking the insulators.

The invention will be readily understood from the following description of one embodiment thereof, reference being made to the accompanying drawings, in which- Figure Ii is a view in side elevation of the preferred form of the invention; 2 is a longitudinal central sectional view of the plug shown in Fig. 1; Fig. 3 is a view in side elevation" of the insulator used in the plug and also shows in cross section the bushing by which it is secured in operative position; Fig. i is a cross sectional View through the insulator and bushing at one stage in the manufacture of the device; and

ig. 5 is a cross sectional view similar to Fig. 4 but showing the parts at a later stage in the process of manufacture.

Referring now to the drawings, 2 indicates the shell or body of the plug which preferably is made from bar stock of box agonal cross section by turning off to the desired diameter the lower part of a piece of stock of the required length, forming a threaded portion 4 on this reduced part and a plain sleeve-like portion 6 below the threaded part 4. The shell is then drilled and reamed to form the bore 8, this bore preferably being cylindrical in shape from its lower end up to a point indicated approximately at a, Fig. 2, and then tapering gradually from this point to the outer end of the shell. This taper is very slow or gradual and is of the character sometimes termed a sticking taper. It will be evident that this shell can be formed automatically in a turrent lathe, all the operations being performed before'the shell is severed from the bar from which it was made, thus requiring but a single handling of this member.

The insulator 10 is formed with a tapered exterior conforming to the tapered bore of, the shell and is held in operative position in the shell by means of a thin bushing 12 which is fitted tightly on to the tapered surface of the insulator, the insulator and the bushing fixed thereon then being forced into the tapered bore of the shell where the frictional engagement of the bushing with the insulator and shell holds these parts in their. proper relationship. v

Unless unusual care is taken in the manufacture of the insulators commonly used in spark plugs, they will not fit accurately the seat in the shell for which they are intended. Most of these insulators are made of porcelain and this material warps in the baking operation; but, even if other materials are used, more than ordinary care must be exercised and special operations must be performed, thus involving additional expense, in order to make the insulators fit the shells properly. For this reason it has been usual to secure the insulators in their shells with flexible packings but this expedient is open to the objection that it produces an unequal distribution of pressure on the insulator and these inequalities are intensified by the sudden changes in temperature to which the plug is subjected. In fact this manner of placed on bushing may be cut holding the insulator and the consequent unequal distribution of strain in the insulator is one of the chief causes of the frequent breakage of these parts. A close examination of any considerable number of i'nsulators of the type shown in the drawings and madeby the usual process, would reveal the fact that a very large percentage of them have neither a true tapered-surface nor a cross sectional outline of exactly circular form. Fig. 4: shows a cross sectional outline that one might reasonably expect to find in an insulator of this type. Where it is necessary or desirable to use insulators subject to these variations I prefer to secure the insulator in the shell by the following method: Assuming a shell to have been completed and an insulator of the proper form and made in the usual manner having been provided, the bushing 12 is next the insulator in the posititon shown in full lines in Fig; 3, and is then forced into the dotted line position shown in Fig. 3 by any convenient mechanism, as for instance, by means from tubing of proper size and preferably is made of some soft metal, such for instance as aluminum or copper. In being forced on to the insulator in the manner just described the bushing will be distorted in accordance with the external shape of the insulator and will be caused to conform to the irregularities of the surface of the insulator. It will be noted that the bushing is forced on to a part of the insulator the smallest diameter of which is greater than the original diameter of the bushing so that a tight fit between the tapered surface of the insulator and the inner surface of the bushing is insured at all points. A transverse section through the insulator and bushing at this time would appear substantially as shown in Fig. 4, the exterior of the bushing being somewhat eccentric due to the irregularities introduced into it by the shape of the insulator. The insulator with the bushing fast thereon is next machined in a suitable lathe or jig, or is acted upon in any other convenient manner, to shape its outer surface to fit the tapered seat in the shell 2. By this operation a true surface is given to 'the bushing 12, and a transverse section through the bushing and insulator after this operation has been completed would appear substantially as shown in Fig. 5. Next, the insulator and the bushing fast thereon are forced into the bore 8 of the shell 2 with suflicient pressure to cause the frictional engagement between the bushing and the shell to hold the insulator securely in the shell. The bushing thus has a gas tight fit both with the insulator and with the shell and it is so shaped in cross section, both transversely and longitudinally, that it compensates for any variaof a suitable die. This tions in shape between the insulator and the seat for the insulator in the shell. A soft metal bushing, such for instance as aluminum, preferably is used since it conforms readily to the shape of the insulator, it is easily machined and it has a surface that enters into a firm frictional engagementboth with the shell and with the insulator. This method insures a firm support for the insulator throughout the portion that is gripped by the bushing and produces a substantially uniform distribution of pressure through the insulator. The bushing also acts somewhat as a cushion for the insulator and relieves to some extent the strains placed upon the insulator due to sudden changes in temperature. It will be observed that hold-- ing the insulator in the shell solely by means of a friction bushing in the manner herein described avoids the use of devices bearing upon the insulator that can be manipulate by the user of the plug to increase the pressure on the insulator with the consequent liability of breaking it. 1

The electrodes employed may be of any suitable form. In the construction shown the sleeve 6 of the shell 2 constitutes one-electrode. The other electrode 14 is made of any of the common compositions or alloys capable of resisting the disintegrating action of'the spark, and is swaged stem 16 that projects through a central bore in the insulator 10. An enlargement 18 of this stem rests in a suitable seat formed in the inner end of the insulator (that is, the end toward the cylinder) and the outer end of this stem is threaded to receive a threadinto an electrode ed cap 20' and a binding screw 22. A fiexible washer tween the top of the insulator and the cap 20.

Having thus described my invention, what I claim as new and desire to secure by Let ters Patent of the United States is:

1. In a spark plug, a shell, an insulator in said shell, and a metal bushing operating solely through frictional engagement with said insulator and shell to hold the insulator 11']. the shell, said bushing being shaped to compensate for variations in shape between the insulator and the seat for said insulator in said shell.

2. In a spark plug, a shell having a bore gradually tapered toward its outer end, an insulator tapered in the same directionas said bore but incapable of fitting said bore accurately, and a soft metal bushing interposed between the tapered surfaces of said insulator and shell and operating solely through frictional engagement with said surfaces to hold the insulator in said shell, said bushing being so shaped in cross section as to form a gas tight connection both with said shell and with said insulator.

3. In a spark plug, a shell having a bore gradually tapered toward its'outer end, an

24 preferably is interposed be:-- 

